1. Field of the Invention
This invention relates to a method and a system for formatting and processing narrow band signals located in a wide band having disturbance. The signals are particularly caused by single-energy gamma-rays, as emitted by a radioactive source, and produced by a radiation detection device, such as a gamma-camera. The disturbance is particularly caused by Compton scattering of said signal.
2. Description of Prior Art
The prior Art is described with particular reference to the field of nuclear medicine imaging, where the present invention is principally, although not exclusively, applied.
Nuclear imaging aims to estimate the spatial and temporal distribution of a radioisotope by detecting its primary radiation. Basic principles of such device is described in a U.S. Pat. No. 3,011,057 to Anger. The ability to produce images by means of scintillation, as described by Anger relies essentially on the possibility of:
(a) selecting only rays lying in a given direction (or range of directions), as PHYSICAL INPUT, by mean of a COLLIMATION device,
(b) converting each gamma ray into light by means of a scintillation CRYSTAL device,
(c) converting light into electrical pulses by mean of an array of PHOTOMULTIPLIERS TUBES positioned behind the scintillation device,
(d) measuring the amount of light produced by a particular scintillation event, which will be equated to the initial energy of each gamma ray, by means of a WEIGHTED SUMMATION circuitry of all photomultiplier tube responses,
(e) determining the position of scintillation events by means of a COMPARISON of each photomultiplier response,
(f) rejecting events having an energy, or SPECTRAL signal lying outside a given energy window, or within a limited set of energy windows, by means of an ENERGY DISCRIMINATOR device,
(g) sending position, referred to as INFORMATION to a FORMATTING DEVICE for immediate display, such as an oscilloscope, for analog storage, such as films, or for digital storage, such as computer memory.
Most modern scintillation cameras also include circuitry for:
(h) energy correction, that is, performing step (d) taking into account differences in the regional response to a given scintillation event and adjusting the output of the weighting summation circuitry accordingly,
(i) linearity correction, that is, event having been corrected for energy, (step h), defining position (step e), taking into account local distortions of the system as compared to an object of known structure and repositioning the events accordingly,
(j) uniformity correction, that is, events having been corrected for energy, (step h), and linearity (step i), formatting spatial information (step g) taking into account local aberrations in sensitivity of the system, adding or subtracting events accordingly.
The energy discriminating capability of windowing a narrow band in the whole energy spectrum, point (f), was a decisive advance for nuclear imaging technology. In fact,any device lacking such capability is unfit for nuclear imaging, would it possess the best of all other characteristics. Nuclear radiography, using image intensifiers for example, failed for this reason. Windowing is imposed by the very nature of gamma ray emission process. A radioactive isotope usually emits within one (or few), quite narrow, specific energy bands. Scattering of gamma rays within matter causes a loss in photon energy and a deviation from its original direction: scattered photons lose their spatial relationship with the source from which they originate. Hence, the mandatory role of energy discrimination whereby scattered photons are denied access to the formatting device. However, because of the intrinsic uncertainties in the measurement of the energy signal, perfect separation of primary and scattered photons, thought theoretically possible, is practically unachievable, the two processes overlapping each other.
The problem of scatter contamination is well known and various methods have been proposed to deal with it. For example, (a) at the expense of a substantial decrease in sensitivity, one can increase primary to scatter ratio by shifting the window towards higher energy; (b) discrimination can also be formed by simultaneous acquisition of images, one on-peak containing both scatter and primary contributions and the other, in a lower part of the spectrum, to estimate scatter, the final result being the subtraction of the two images; (c) numerical image enhancement addresses the scatter contamination problem by applying a filter according to the point spread function in a scattering medium; or, (d) more recently, as described in U.S. Pat. No. 4,575,810, a modification of the acquisition, applying an energy dependent, weighting factor on the pulse level, can decrease the effect of scatter on the imaging process.
In this latter method, a weighted image is generated which is dependent on the energy of the pulse to be processed. It takes into account the fact that both the signal-to-noise ratios and the modulation transfer functions are energy dependent. The weighting factors are selected in order to optimize a single figure-of-merit of the overall response of the system to a point source in water and at fixed distance from the camera. The performance of such a method is thus limited by the fact that the weighting factors are optimized for a very particular situation.
In summary, all the methods available in the prior art, have various limitations because of sensitivity, noise or reliance on singular experimental setups that do not take into account the large variability in the characteristics (size, geometry, diffusing media . . . ) of the distribution of the photon source.
In the present invention, a method and a system is provided in which the transformation to be applied is based and evaluated on the data to which the transformation is to be applied: it varies from case to case, taking into account actual difference in the distribution of the source. In that sense, the present invention addresses the problem of the effect of scatter on the actual images, while most of the previous methods address the effects of scatter on photon energy or on system point response.